This invention relates to a method of converting an oxygenate feedstock to an olefin product. In particular, this invention is to a method for converting an oxygenate feedstock to an olefin product by contacting a silicoaluminophosphate catalyst with a feedstock, followed by stripping a portion of the catalyst and returning the stripped catalyst to the reaction zone.
Olefins, particularly light olefins, have been traditionally produced from petroleum feedstocks by either catalytic or steam cracking. Oxygenates, however, are becoming an alternative feedstock for making light olefins. Particularly promising oxygenate feedstocks are alcohols, such as methanol and ethanol, dimethyl ether, methyl ethyl ether, diethyl ether, dimethyl carbonate, and methyl formate. Many of these oxygenates can be produced from a variety of sources including synthesis gas derived from natural gas; petroleum liquids; carbonaceous materials, including coal; recycled plastics; municipal wastes; or any appropriate organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum source for light olefin production.
One way of producing olefins is by the catalytic conversion of methanol using a silicoaluminophosphate (SAPO) molecular sieve catalyst. For example, U.S. Pat. No. 4,499,327 to Kaiser, discloses making olefins from methanol using any of a variety of SAPO molecular sieve catalysts. The process can be carried out at a temperature between 300xc2x0 C. and 500xc2x0 C., a pressure between 0.1 atmosphere to 100 atmospheres, and a weight hourly space velocity (WHSV) of between 0.1 and 40 hrxe2x88x921.
However, during conversion of oxygenates to light olefins, by-products are also formed. Representative by-products include alkanes (methane, ethane, propane, and larger), C4+ olefins, aromatic compounds, and carbon oxides. Carbonaceous deposits on and within the catalyst materials (also referred to as xe2x80x9ccokexe2x80x9d) are also formed in the process. As the amount of these carbonaceous deposits increases, the catalyst begins to lose activity and, consequently, less of the feedstock is converted to the desired light olefin products. At some point, the build up of these carbonaceous deposits causes the catalyst to reduce its capability to convert the oxygenates to light olefins. Once a catalyst becomes deactivated, it must be removed from the reaction vessel and replaced with activated catalyst. To reduce catalyst costs, activated catalyst is obtained by removing the carbonaceous deposits from the deactivated catalyst. This process is typically referred to as regeneration, and typically takes place in a vessel called a regenerator.
Catalyst regeneration is typically accomplished by periodically removing the deactivated catalyst from the reactor vessel, burning off the carbonaceous material in the regenerator to reactivate or regenerate the catalyst, and returning the regenerated catalyst to the reactor. Prior to entering the regenerator, any volatile organic components which may be adsorbed onto the catalyst or located within its microporous structure may be stripped off using a substantially inert stripping gas e.g., steam. The regenerated catalyst is then returned to the reactor.
Recently, it has been shown that catalyst selectivity to light olefins increases if the level of coke on the catalyst is controlled in some manner. One way of controlling the rate and manner in which the catalyst accumulates coke is taught by U.S. Pat. No. 6,023,055 to Lattner et al, and assigned to the assignee of the present application. Lattner et al. discloses a process whereby the oxygenate exposed catalyst exiting the reaction zone is separated into two portions. One portion is returned to the reaction zone, and the other portion settles into a stripping zone prior to entering the regenerator.
Methods are needed which will maintain a desired level of coking on the molecular sieve catalyst during the conversion of oxygenates to olefins. The desired level of coke is that which optimizes light olefin selectivity and/or decreases the rate of deactivation. It is, therefore, an object of the invention to control the amount and manner at which coke deposits on the catalyst.
The present invention controls the manner in which coke deposits on catalysts, particularly small-pore molecular sieve catalysts. In achieving this object and other objects of the invention, including a process for the production of olefin product from an oxygenate-containing feedstock, the invention comprises: exposing a molecular sieve catalyst to an oxygenate-containing feedstock in a reaction zone under conditions effective to convert the oxygenate-containing feedstock to an olefin product; stripping at least a portion of the exposed catalyst with a stripping gas; and returning at least a portion of the stripped catalyst to the reaction zone without regeneration. The intermittent removal of hydrocarbons adhered to the catalyst during the stripping process provides a degree of operational control so as to control the manner in which coke, is deposited on the catalyst. The result is an increase in product selectivity to light olefins and an increase in catalyst lifetime. Following the stripping process at least a portion of the stripped catalyst is returned to the reaction zone thereby repeating the process, which is preferably a continuous process. Optionally, a portion of the stripped catalyst may be directed to a regenerator prior to its return to the reaction zone. However, at least a portion of the stripped catalyst is not regenerated before returning to the reaction zone.
Hydrocarbons that may be adhered to the catalyst include, but are not limited to, oxygenates, aromatics, parafins, and olefins. Preferably, the stripped catalyst contains less than 10% of the hydrocarbons by weight, more preferably less than 5%, even more preferably less than 2%, and most preferably less than 1% of the hydrocarbons by weight. Substantially all of the hydrocarbons are removed from the stripped catalyst when less than 1% by weight of the hydrocarbons remain on the stripped catalyst exclusive of coke. Coke is defined as hydrocarbons that are not effectively stripped from the catalyst. Aromatics and substituted aromatics are examples of coke in this application.
Another feature of the invention is the ratio of the catalyst""s exposure time in the reaction zone to the time the exposed catalyst is being stripped. Preferably, the ratio is from 1:1 to 20:1 and more preferably greater than 20:1. Such ratios intend to show that the catalyst is exposed to feedstock for relatively shorter periods of time prior to stripping relative to conventional methods.
The present invention controls the manner in which coke is deposited on the catalyst by stripping the coke precursor molecules away from the catalyst to a greater extent relative to a catalyst that was not intermittently stripped of hydrocarbons. Therefore, the catalytic activity of the catalyst unexpectedly is maintained at optimal levels for longer periods of time because the catalyst is not as readily deactivated. The process of the invention also increases the selectivity to light olefins. Moreover, since the overall coke forming process is exothermic, a reduction in the amount of coke produced reduces the amount of heat produced thereby reducing the heat removal requirements of the process equipment.